Question

A common gate amplifier represents a _____________.

• A. voltage controlled voltage source
• B. current controlled current source
• C. voltage controlled current source
• D. current controlled voltage source

Hint:

Compare the common gate FET stage to its BJT twin, the common base amplifier: both are current buffers, not voltage amplifiers driven by a gate or base voltage.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks what kind of two-port model best describes a common gate (CG) amplifier built with a MOSFET or JFET. To answer this, compare how the CG stage treats its input and output signals with the standard controlled-source models used in amplifier theory.

Step 2: Key Formula or Approach:
In a common gate stage, the signal is fed into the source terminal and taken out at the drain, while the gate is held at signal ground. Because the source terminal has low input impedance (close to \(1/g_m\)) and the drain has high output impedance, the stage behaves as a current buffer: the current that enters the source leaves the drain almost unchanged (current gain close to \(1\)), while a voltage gain still appears because a low input resistance is driven by a much higher output resistance path. A block with a low-impedance current input and a high-impedance current output, where the output current tracks the input current, is modeled as a current controlled current source (CCCS).

Step 3: Detailed Explanation:
Compare this with the other three single-stage FET amplifiers.
A common source (CS) stage has high input impedance (voltage input at the gate) and converts that gate to source voltage into a drain current through \(g_m\); this is the classic voltage controlled current source (VCCS), which is option (C), not the CG stage.
A common drain (CD) stage, also called a source follower, has high input impedance and low output impedance, and its output voltage follows the input voltage with gain close to \(1\); this is a voltage controlled voltage source (VCVS), option (A), again not the CG stage.
Option (D), current controlled voltage source, describes a transimpedance amplifier, which needs an extra feedback resistor around an active network to turn a current into a voltage; a plain single-transistor CG stage has no such feedback resistor.
The CG stage instead takes the input current at its low-impedance source terminal and reproduces it at the high-impedance drain, which is exactly the behavior of a current controlled current source.

Step 4: Final Answer:
The common gate amplifier is best modeled as a current controlled current source, so the correct option is (B).